Gas Furnace Makes Noise when it shuts off


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Old 01-30-10, 06:16 AM
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Gas Furnace Makes Noise when it shuts off

Temps outside are 0 to 10. (MN)
Newly remodeled house, so furnace is 4 yrs old.
Gas Payne 90% HE furnace with zone control, thermostat upstairs and on mail floor. Furnace is located in basement.
Digital Honeywell thermostats set at 70.

Furnace is cycling on for about 8 min, off every 2-4 minutes (which also seems too frequent???).
When it shuts off, there is a metal on metal noise (sounds like a sword being pulled out of a metal holster), lasts about 1/2 second. I think its temperature related since its not as loud if I force the furnace to turn off, and louder when I let it run, heat up, then cycle off.
And for some reason, when I stand right next to the furnace, I can barely hear the noise. I think its loudest when standing near a cold air return right above the furnace on the main floor.

So, what is the noise? How can I fix it? And is it cycling too often?

Thanks!
 
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Old 01-30-10, 07:42 AM
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I had one making a noise like that. The fan motor was bad. If there is drive belt, check that first.
 
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Old 01-30-10, 09:52 AM
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Your run and off cycle frequency can be simply related to how well your house is sealed against drafts, and insulated.

If the furnace comes back on quickly, with no rise in temp at the stat, that could indicate what I just said. Also, for the same reason, the furnace will run longer. But if the furnace runs short, and is off short, with no heat gain at the stat, it can mean the furnace was oversized, along with cold temps out.

Or - frequent both on and off cycles can also mean stat adjustment for anticipator(in mechanical stats) or cycles per hour or swing temp dipswitches (in digitral stats) are not set to their optimum.

You can't isolate this sound by opening the blower and then the big burner compartments? There are only two things that move and that be the inducer fan and blower fan. Could be a bearing, or a blade/blower wheel hitting the housing under certain temperature conditions.
 
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Old 01-30-10, 05:59 PM
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Hocum's Razor

the simplest solution is often the answer. Turns out it was a squeeky vent gate on an old cold air intake that would swing a bit when the furnace shut down and the air pressure was moving gate, which squeeked.
 
 

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